Personally, I do not think email is becoming or will become a legacy medium. The form of email between professor and student holds a standard of professionalism that texting, Facebook, and WhatsApp lack.
Students in high school were never taught how to properly email teachers which is why they enter college clueless about email etiquette. However, the high school teacher-student relationship is probably more relaxed and casual compared to the college professor-student relationship. Whenever I would email teachers in high school though I would always try to keep the email as respectful as possible because I knew they would be taking time out of their day to read and respond to me. I have never thought about casually addressing a teacher in email because I understand there is a professional relationship that still needs to be maintained.
I think especially in college that professors need to be addressed in correct email etiquette because as our educators they need to be shown respect in their rightful positions of authority and expertise. Email etiquette also shows that students know the proper way to address others in a professional manner, such as potential employers. Professors are used to writing in such a formal manner because they have to address other professors and coworkers with the same respect when making a special request. For students, they are used to addressing each other via texts in shorthand notation so formal emails in a letter template seem almost foreign.
Hopefully email as a media stays relevant because texting professors or Facebook messaging them would seem really weird and too casual for an educational setting.
It’s interesting (and useful) to think about how the student-prof (or h.s. student-teacher) email is an extension of the classroom situation here. Profs see their students less frequently—and may know them less well?—so perhaps an excessively casual email can seem rude or presumptuous to them (us).